16.11.2012 23:42, Daniel J. Greenhoe wrote:> It seems the most "common" definition of a topology is that T is a topology on a set X if> 1. empty set is in T and> 2. X is in T and> 3. A and B are in T ==> A intersection B is in T and> 4. {A_i} in T ==> Union A_i is in T.>> But some authors imply that only 3 and 4 are necessary for the definition of a topology. For example, Kelley ("General Topology", 1955, page 37) only uses 3 and 4 and says that these imply X is in T. McCarty ("Topology...", page 87) says 1 and 2 are "completely unneeded".>> My question is, is it really possible to exclude 1 and 2 from the definition such that 3 and 4 alone imply 1 and 2?>> Suppose X:={x,y,z} and T:={ {x},{y},{x,y} }.> Then T satisfies conditions 3 and 4, but yet X is not in T.> So how is it possible to exclude 3 from the definition of a topology?

By convention, the intersection of zero number of subsets of X is the whole space X. Similarly, the union of zero number of subsets of X is the empty set.